Docs/mm/damon/design: document quota goal self-tuning

Update DAMON design doc to explain the quota goal self-tuning, which can
be used by setting the goal's metric to metrics that kernel can
self-retrieve.

Link: https://lkml.kernel.org/r/20240219194431.159606-16-sj@kernel.orgSigned-off-by: SeongJae Park <sj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Documentation/mm/damon/design.rst

@@ -398,12 +398,28 @@ Aim-oriented Feedback-driven Auto-tuning
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 Automatic feedback-driven quota tuning.  Instead of setting the absolute quota
-value, users can repeatedly provide numbers representing how much of their goal
-for the scheme is achieved as feedback.  DAMOS then automatically tunes the
+value, users can specify the metric of their interest, and what target value
+they want the metric value to be.  DAMOS then automatically tunes the
 aggressiveness (the quota) of the corresponding scheme.  For example, if DAMOS
 is under achieving the goal, DAMOS automatically increases the quota.  If DAMOS
 is over achieving the goal, it decreases the quota.
 
+The goal can be specified with three parameters, namely ``target_metric``,
+``target_value``, and ``current_value``.  The auto-tuning mechanism tries to
+make ``current_value`` of ``target_metric`` be same to ``target_value``.
+Currently, two ``target_metric`` are provided.
+
+- ``user_input``: User-provided value.  Users could use any metric that they
+  has interest in for the value.  Use space main workload's latency or
+  throughput, system metrics like free memory ratio or memory pressure stall
+  time (PSI) could be examples.  Note that users should explicitly set
+  ``current_value`` on their own in this case.  In other words, users should
+  repeatedly provide the feedback.
+- ``some_mem_psi_us``: System-wide ``some`` memory pressure stall information
+  in microseconds that measured from last quota reset to next quota reset.
+  DAMOS does the measurement on its own, so only ``target_value`` need to be
+  set by users at the initial time.  In other words, DAMOS does self-feedback.
+
 
 .. _damon_design_damos_watermarks: